Try these steps in sequence to resolve the sign-in issues:
Verify that you are using the correct email address and password combination. If you have more than one Amazon account with the same email address but different passwords, use the corresponding password for each account. If you have an existing seller account with Amazon, you must use a unique email address and storefront name when creating your new account to sell on Amazon.com.au.
Ensure that there are no extra spaces in your password. This can happen when you copy and paste your password from somewhere else.
Verify that you have entered the most recent Two-step verification code that you have received, if you are prompted to enter one. Older codes will not work. For more information, refer to the help page Two-step verification FAQ.
To reset your Two-Step Verification follow the steps here
Clear your browser cookies and cache or try logging in with a different browser or device.
Use our Password Assistance page to verify whether the email address that you are using is the one registered in our system.
If yes, change your password by using the Password Assistance page.
Log on to Seller Central using your new email address and password combination.
If these steps do not resolve the sign-in issue, it is likely that your email address and password combination is not associated with an active Seller Central account. If you did not deactivate your account, contact us.
Additional sign-in troubleshooting
I forgot my password.
Use our Password Assistance page to change your password.
Do not create a new Seller Central account.
Note: If you change your password and are using something other than Seller Central (for example, a SOAP server or the Amazon Merchant Transport Utility) to submit product-related and order-related data, you might need to reconfigure those services with your new password.
How do I access my company's Seller Central account?
If your company has already registered with Seller Central, your company's account administrator can set up a user account for you.
If your company has not signed up to use Seller Central, visit Amazon Services to find out how you can get started.
How do I get technical support for my account?
If you already have a Seller Central account, contact us using Seller Central. You must sign in first.
What happens if I do not receive my Two-step verification code via SMS text message on my mobile?
Click the "Didn’t receive the code?" link on the web page where it asks you to enter your code. This will list any back-up methods that you have designated when you set up your account. If you do not receive the SMS text, you can opt for a voice call to your registered mobile number. If you do not receive a code during registration, check that the provided phone number has no typos and includes a region code and that your mobile is able to receive SMS text messages.
I already have a selling account in another Marketplace, can I use my existing account to sell in Australia?
We do not currently have linked accounts in Australia. If you have an existing seller account with Amazon, you must use a unique email address and storefront name when creating your new account to sell on Amazon.com.au.